Field of the Invention
The present invention relates generally to epoxy resins and more particularly to accelerator compositions used in the curing of epoxy resins.
Background of the Invention
N-aminoethylpiperazine (“AEP”) is widely used in conjunction with polyetheramines, e.g. JEFFAMINE® D-230 amine and JEFFAMINE® T-403 amine, to serve as an accelerator for increasing the polymerization rate of epoxy resins cured with such hardeners (JEFFAMINE is a registered mark of the Huntsman Corporation of The Woodlands, Tex.). AEP has provided some of the highest exotherm temperatures seen when used to cure epoxy resins. Providing high exotherm temperatures can be advantageous to promote increased curing but can lead to polymer degradation if unchecked. Therefore, the amount of AEP used in such a reaction needs to be closely monitored. Because the equivalent weights of AEP typically differ from those of the other epoxy hardeners (such as amine based hardeners) used for epoxy resin curing, calculations must typically be done each time a different accelerator level is used (of reactive amine containing accelerators) to adjust the reactivity of the epoxy formulation. For many users, this is cumbersome and can lead to errors.
Other accelerators of amine cured epoxy blends exist, but they each have particular drawbacks that can make them unsuitable for certain applications. For instance, phenolic accelerators are often solids and contribute undesired color or ultraviolet light sensitivity to the final formulation. Widely used liquid accelerators, e.g. nonyl phenol, mono-nonyl phenol (MNP), dinonyl phenol etc., also serve as plasticizers, significantly and sometimes undesirably decreasing the glass transition temperature (Tg) of resin systems into which they are incorporated at levels high enough to provide significant acceleration. Additionally, the accelerating effect diminishes as increasing levels of MNP are used since the reactant group concentrations (amine and epoxide) are diminishing.
Tertiary amines such as ethanolamine derivatives that contain high levels of hydroxyl groups, such as triethanolamine, methyldiethanolamine, dimethylethanolamine, etc., have been effectively used as accelerators but since they remain as small molecules that do not react into the polymer network, they too are known to cause significant decreases in Tg. In addition, several such ethanolamine derivatives are now regulated by governmental authorities due to their potential use as chemical weapons precursors.